We actually have been having this discussion in several groups over the last year. The consensus is that there's a lot of truth in it *at least in terms of OUT polys.
There is a similar observation in regards to discrimination(but not specifically poly) that has been made. That is that the more different "minority statuses" you fit into, the less likely you are to be open regarding any that *can be hidden. For example, minority race, minority gender, minority sexual preference, minority lovestyle=more likely to be in the closet than someone who is majority race, majority gender, majority sexual preference.
There does definitely appear to be some relationship to having financial security-but not so much as to be in the "media spotlight" (at least in certain spheres like politics) and safety.
Personally I have also noticed that there seems to be more "out" people in higher educated spheres. The lower educated spheres seem to be more closeted. I dont know if that one is happenstance or not as its just a personal observation.
"Love As Thou Wilt"